1. Field of the Invention
The invention relates to a fuel cell membrane-electrode assembly and a production method therefor.
2. Description of the Related Art
Fuel cells directly convert chemical energy into electric energy by supplying a fuel and an oxidant to two electrically connected electrodes and electrochemically causing oxidation of the fuel. Unlike thermal power plants, the fuel cells are free from the restrictions of the Carnot cycle, and exhibit high energy-conversion efficiency. Ordinarily, the fuel cells are constructed by stacking a plurality of unit cells that have, as a basic structure, a membrane-electrode assembly formed by sandwiching an electrolyte membrane between a pair of electrodes. In particular, a solid polymer electrolyte fuel cell incorporating a solid polymer electrolyte membrane as an electrolyte membrane is drawing attention particularly as a portable power source or a power source of mobile bodies due to its advantages of being easy to miniaturize, being operable at low temperature, etc.
During the normal power generation of a solid polymer electrolyte fuel cell that uses hydrogen as a fuel and oxygen as an oxidant, the reaction of the formula (1) progresses at a fuel electrode (anode).2H2→4H++4e−  (1)
The electrons generated in the reaction of the formula (1) move through an external circuit, and reach the oxidant electrode (cathode) after working in a load provided outside. The protons generated in the reaction of the formula (1) move in a water-hydrated state within a solid polymer electrolyte membrane from the fuel electrode side to the oxidant electrode side.
At the oxidant electrode, on the other hand, the reaction of the formula (2) progresses.4H++O2+4e−→2H2O  (2)
That is, as the whole cell, the following reaction progresses.2H2+O2→2H2O  (3)
In order to accelerate the reactions of the formulas (1) or (2), each of the electrodes, that is, the fuel electrode and the oxidant electrode, is provided with an electrode catalyst. Generally, an electrode catalyst has a construction in which a catalytically active substance, such as a metal catalyst particle or the like, including particles of platinum, a platinum alloy, etc., is supported on an electrically conductive material such as a carbon particle or the like. Metal catalysts, such as platinum or the like, that are used as electrode catalysts are very expensive. In order to put fuel cells into practical use, there is a demand for development of a fuel cell in which the utilization rate of the catalyst is improved so as to exhibit excellent power generating performance while minimizing the amount of the catalyst that needs to be employed.
Examples of the method of improving the utilization rate of the metal catalyst include the particle size reduction of the metal catalyst particle. By reducing the particle diameter of the metal catalyst particle, the exposed surface area of the metal catalyst increases if the amount of the metal catalyst employed remains the same. In this manner, the utilization rate of the metal catalyst can be heightened. However, small metal catalyst particles are difficult to disperse, and very easily aggregate. Therefore, the particle size reduction finds it difficult to effectively increase the exposed surface area. It is known that the particle diameter of platinum particles supported on carbon particles is generally about 2 to 3 nm.
The reactions of the formula (1) and the formula (2) progress at a three-phase interface where reactant gases, that is, the fuel and the oxidant, that is, protons (H+) and electrons (e−), can be given and received. That is, it is important to dispose the catalytically active substance near the three-phase interface in order to improve the catalyst utilization rate. For example, Japanese Patent Application Publication No. JP-A-2002-151088 describes a technology of efficiently forming places of reaction where the electrode reactions progress and therefore improving the utilization rate of platinum through the use of a catalyst ink that has been prepared by dispersing platinum black in a solution that contains a fluorine-containing ion exchange resin and a fluorine-containing compound solvent so that the viscosity of the solution is 100 to 20,000 cP.
As described above, a metal catalyst, such as platinum or the like, is ordinarily incorporated in such a fashion that the metal catalyst is supported on an electrically conductive particle, such as a carbon support or the like including a carbon particle or the like. The catalyst layer formed by using such a catalyst-supporting carbon particle becomes a thick layer corresponding to the bulk height of the carbon particle. Most reactions occurring at the catalyst layer progress on the metal catalyst that is present near the interface between the electrolyte membrane and the catalyst layer or present near the interface between the catalyst layer and the gas diffusion layer. Therefore, on a thick catalyst layer as mentioned above, the catalyst utilization rate tends to be very low, that is, the amount of the metal catalyst effectively contributing to the electrode reactions is ⅓ to ½ of the amount of catalytically active substance contained in the catalyst layer. This is because in a thick catalyst layer, the diffusion of the reactant gas, protons and electrons is likely to be impeded.
Therefore, in some cases, a metal catalyst particle, such as platinum black or the like, is not supported on a support, such as carbon particle or the like, but is simply employed. However, such a support-less metal catalyst particle very easily aggregates, depending on the particle diameter thereof. Thus, it is very difficult to form a catalyst layer in which a metal catalyst particle is highly dispersed.
JP-A-2002-151088, mentioned above, teaches that the state of aggregation of the platinum black particle can be stabilized in a desired state by adjusting the viscosity of the catalyst ink containing a fluorine-containing ion exchange resin, a fluorine-containing compound solvent and platinum black, within the range of 100 to 20,000 cP. However, although JP-A-2002-151088 teaches that a preferable particle diameter of the platinum black is 0.01 to 30 μm, the particle diameter of the platinum black actually used in a working example is 3 μm. Such a large-particle diameter platinum black may be able to be dispersed to a high degree, but is not able to sufficiently improve the catalyst utilization rate due to its small surface area. Furthermore, the ratio between the platinum black and the fluorine-containing ion exchange resin recommended in JP-A-2002-151088 (platinum black:fluorine-containing ion exchange resin=0.2:0.8 to 0.9:0.1 (mass ratio)) may suffer from large catalyst layer resistance and therefore decline of the cell output because the amount of fluorine-containing ion exchange resin is rather large. Furthermore, in this technology, it is sometimes difficult to fabricate electrodes where the electrode reactions efficiently progress.
Besides, the use of a support-less metal catalyst particle has a merit of being free of the oxidation corrosion of the carbon support in a state where the fuel is lacking and free of the various problems resulting from the oxidation corrosion of the carbon support. The carbon material, such as carbon particle or the like, that is used as a catalyst support that supports metal catalyst particles is a material that has such a large surface area as to support many metal catalyst particles. Generally, the carbon material having large surface area (generally, 300 m2/g or greater) which is used as a catalyst support as mentioned above is low in oxidation corrosion resistance.
It is known that the cell characteristic declines if a state where the fuel, such as hydrogen or the like, becomes lacking (hereinafter, referred to as “fuel deficiency”) occurs due to some cause, for example, the closure of a gas channel, flooding, etc. In a unit cell where abnormality has occurred in the state of supply of the fuel and therefore fuel deficiency has occurred, the protons and the electrons produced by the oxidation of the fuel become insufficient. In order to make up for the insufficient protons and electrons, the electrolysis of water present in the fuel electrode or water held in the electrolyte membrane or the oxidant electrode (H2O→2H++2e−+½O2) progresses in this unit cell. At this time, the electrical potential of the fuel electrode of the unit cell rises to the electrolytic potential of water, resulting in a reversed potential state in which the electrical potential of the fuel electrode (anode) and the electrical potential of the oxidant electrode (cathode) are reversed.
The electrical potential of the fuel electrode remains stable as long as protons and electrons are sufficiently supplied by the electrolysis of water at the fuel electrode. However, when sufficient supply of protons and electrons is not secured only through the electrolysis of water, the electrical potential of the fuel electrode further rises so that the oxidative reaction of the carbon material that constitutes the electrode (C+2H2O→CO2+4H++4e−) occurs, supplying electrons and protons. This oxidation corrosion of the carbon material decomposes and depletes the carbon material.
The decomposition, depletion or the like of the carbon material that is an electrically conductive material lowers the electrical conductivity, and causes increased resistance due to bad contact. Besides, the decomposition, depletion or the like of the carbon material that is a water-repellent material lower the water repellency of the cell, and facilitates the occurrence of the flooding, which inhibits the supply of the reactant gas. Furthermore, due to the decomposition or depletion of the carbon material supporting the metal catalyst particle, metal catalyst particles fall apart or move, so that the effective catalyst surface area decreases. Thus, due to the oxidation corrosion of the carbon material, the power generation performance of the fuel cell considerably declines, and stable power generation performance cannot be delivered.
Furthermore, even if the fuel deficiency state is resolved, there may occur a case where during the transition from the fuel deficiency state to a normal power generating state, a partial cell reaction caused by non-uniform distribution of the fuel on the fuel electrode side results in a local high electrical potential on the oxidant electrode. In consequence, the oxidation corrosion of the carbon material occurs on the oxidant electrode side as well, and concomitantly various problems as mentioned above occur on the oxidant electrode side as well.
Because the oxidation of the carbon material in the fuel electrode and the oxidant electrode as mentioned above is a irreversible reaction, the resolution of the fuel deficiency will not bring the performance of the fuel cell back to the state preceding the fuel deficiency. Examples of the technologies for solving problems resulting from the fuel deficiency state include a technology described in Published Japanese Translation of PCT Application, JP-T-2003-508877, a technology described in JP-A-2004-22503, etc.
Furthermore, the carbon material has catalytic activity for the reaction that produces hydrogen peroxide from oxygen and hydrogen. Therefore, hydrogen peroxide is sometimes produced from oxygen and hydrogen, depending on the state within the fuel cell. Hydrogen peroxide produces radicals, a strong oxidant, and thus degrades a cell-constituting material such as the electrolyte resin, and the like.
It is considered that these problems arising from the carbon material will be able to be prevented by incorporating a catalytically active substance without supporting it on carbon particle. For example, JP-A-2005-294264 proposes a membrane-electrode assembly having a cathode catalyst layer that contains a mixture of platinum black, and platinum supported on a carbon support, for the purpose of restraining the corrosion of the carbon support and the accompanying decline of the catalyst performance.
Besides, Published Japanese Translation of PCT Application, JP-T-2002-525812, proposes an electrode produced by using a screen printing paste that contains platinum black, a binder polymer and a high boiling point solvent. However, because the screen printing paste is required to have high boiling point and high viscosity, small-particle diameter platinum black is likely to thermally aggregate in the paste. Therefore, in the case where small platinum black particles in the nano-size are incorporated in a non-supported state, the utilization rate thereof as a catalyst is likely to decline.